Amorphous precipitated siliceous pigments and improved process for producing such pigments

ABSTRACT

A process for producing precipitated silicic acid pigments having new and improved properties is disclosed. The pigments are produced by the simultaneous introduction of a solution of an alkali metal silicate and an acid, such as sulfuric acid, into an aqueous receiving medium which contains a salt or electrolyte. The pH of the aqueous receiving medium is adjusted prior to the introduction of the acid and silicate so that it is the same or equal to that pH at which the precipitation of the pigment is effected. The electrolyte is preferably a metal salt of the acid used for acidulation of the silicate. The acid and silicate are added to the receiving medium at a rate such that the precipitating pH is maintained constant. The products of the invention are characterized by their high abrasiveness and have a relative cleaning ability or scale (RCS) equivalent to high grade phosphates used as polishing agents in toothpastes and a superior fluoride compatibility to that of said phosphates. The silicates used in the process are preferably those which are partially polymerized and have an SiO 2  /Na 2  O ratio of at least 2.0.

This is a continuation of application Ser. No. 403,129, filed Oct. 3,1973, now abandoned.

The present invention relates to silicic acid pigments and to a uniqueprocess for producing synthetic precipitated silicas having a uniquecombination of physical and chemical properties.

As known in the art, precipitated silicic acid pigments can be preparedby the acidulation of an aqueous silicate solution with an acid such assulfuric acid, hydrochloric acid, etc. Examples of prior techniqueswhich involve the acidulation of a silicate solution to producesiliceous pigments are disclosed in U.S. Pat. Nos. 3,110,606 and3,582,379. In general it is known that the nature or characteristics ofthe above discussed products, sometimes herein referred to as silicas,depend on the specific reaction conditions employed, as for example, theprecipitating pH, the reaction temperature, etc. Notwithstanding thisfact, prior known pigments are characterized by, and have, the followingproperties: high structure, high wet cake moisture content, high oilabsorption, low valley abrasion, high surface area and low pack density.In this regard, and due to properties such as high oil absorption, highsurface area, etc., the pigments have been widely used as reinforcingpigments in rubber, in the manufacture of paper, as moistureconditioners and the like.

The high wet cake moisture content is disadvantageous however, in thatthe drying and filtration rates are increased, thus increasing theoverall cost of the final product. For example, in the conventionalproduction of silicic acid pigments (as defined above) the wet cakemoisture content of the product (following filtration of theprecipitated reaction mass) is approximately 82%. This means that therecan be recovered only 18 parts of dry pigments from 100 parts of wetcake.

Further, the low abrasiveness of the known silica and silicate pigmentsrenders them unsuitable for many uses. For example it is well known thatconventional synthetic precipitated silicas are unsuitable as polishingand abrasive agents in toothpaste compositions. See German Pat. No.974,958; French Pat. No. 1,130,627; British Pat. No. 995,351; Swiss Pat.No. 280,671 and U.S. Pat. No. 3,250,680. U.S. Pat. No. 3,538,230specifically discloses that known amorphous silicas such as precipitatedsilicas, pyrogenic silicas and aerogels are unsuitable for dentifriceuse because of their initial small particle size and because of the easein which they break down into small particle sizes which result in poorcleaning ability.

In this regard, a primary function of abrasive or polishing agents intoothpaste is to remove stains, food debris and bacterial plaque fromthe tooth surface. Ideally the polishing agent should provide maximumcleaning action at acceptable abrasive levels and must be compatible atloadings of from about 15% up to 50% with the other toothpaste formulaingredients. Examples of known polishing agents include aluminas,thermosetting resins (as, e.g., melamine-formaldehyde resins), zirconiumsilicates and various phosphate salts or compounds such as betatricalcium orthophosphate. Specific examples of phosphate polishingagents are disclosed in U.S. Pat. Nos. 3,169,096; 3,359,170 and3,442,604.

In summary the present invention relates to the production of finelydivided precipitated silicas which, because of their new and uniqueproperties, can be used as a polishing agent in toothpaste compositions.In its broadest aspect, the invention is based on the discovery thathigh abrasion precipitated silicas, having superior fluoridecompatibility and cleaning properties equivalent to the aforementionedphosphates, can be produced by the simultaneous introduction of an acidand an alkali metal silicate into an aqueous reaction medium whichcontains an electrolyte, such as sodium sulfate, and thereaftereffecting the precipitation of the finely divided pigment at a constantpH. Significant process variables include the SiO₂ /Na₂ O mole ratio ofthe alkali metal silicate and the initial pH of the reaction medium.With regard to the latter, the pH of the aqueous reaction medium must beadjusted prior to the introduction of the acid and silicate so that itis the same or equal to that pH at which the precipitation of thepigment is carried out. As will be discussed in more detail hereinbelow,the salt or electrolyte employed or added to the reaction medium at theoutset is preferably a metal salt of the acid used for acidulation. Forexample, if sulfuric acid is employed, the electrolyte would comprise ametal sulfate such as sodium sulfate. Preferred electrolytes comprisesodium, lithium, potassium, and ammonium sulfate when the acid issulfuric acid. If sodium silicate is acidulated with sulfuric acid thensodium sulfate would be the preferred electrolyte.

As indicated, the product of the invention has a unique combination ofphysical and chemical properties including high abrasion and a cleaningability or action equivalent to that of known phosphate and othercleaning or polishing agents. This was truly unexpected and, in part,embodies the concept or discovery upon which the instant invention isbased.

It is accordingly a general object of the present invention to provide aprocess for producing precipitated silicic acid pigments having a uniquecombination of physical and chemical properties.

Another and more particular object is to provide improved syntheticamorphous precipitated silicas which have particular utility for use asan abrasive or polishing agent for dentifrice compositions.

A further object is to provide a novel process for producingprecipitated amorphous silicas which have unexpected properties ascompared to conventionally known precipitated pigments, such propertiesincluding low structure, high abrasion, high pack density, and highcleaning action in toothpastes.

The manner in which the foregoing and other objects are achieved inaccordance with the present invention will be better understood in viewof the following detailed description which discloses particularlyadvantageous method and composition embodiments for illustrativepurposes.

As discussed above silicic acid pigments are conventionally produced bythe acidulation of an aqueous silicate solution with an acid. In otherwords a silicate solution is charged to a reactor with the solution thenbeing acidulated by the introduction of an acid. More recently however(and before turning to the specific details of the invention) there hasbeen developed, as disclosed in U.S. application Ser. No. 285,966, aprocess for producing siliceous pigments having improved propertieswherein a salt or electrolyte is employed to prepolymerize an alkalimetal silicate solution prior to its acidulation with an acid. Inaccordance with the teachings of this application an alkali or alkalineearth salt is first added to a solution of the alkali metal silicate,e.g., sodium silicate, with the latter being pre-polymerized by theaddition of the salt. The said silicate solution is then heated to atemperature in the range of from 100° to 200°F, preferably on the orderof from about 150°-175°F. The acidulating agent or acid, e.g., sulfuricacid, is next charged to the reactor until the precipitation of thesilicate is complete. The reaction or precipitation is carried out at apH in the range of from about 6.5 to 11.0. At the end of theprecipitation, an excess of the acid is preferably added to bring the pHof the pigment within the range of from about 5.5 to 6.5 and thereaction mass is filtered, washed and dried. In a first methodembodiment disclosed by this application, the entire solution of thesalt induced polymerized silicate is initially charged to reactor. Inanother embodiment one-half to two-thirds of the silicate is initiallycharged to the reactor with the remaining silicate being added with theacid.

In contrast, in accordance with the present invention an aqueousreceiving medium, i.e., water, is first charged to the reactor. Theelectrolyte is then added to the water and the pH of thewater-electrolyte media is adjusted such that it is the same at whichthe precipitation of the pigment is to be carried out. In the practiceof the invention the precipitating pH should be in the range of 8 to10.4. Thereafter, aqueous solutions of the acid and silicate (as to bediscussed in more detail hereinbelow) are added simultaneously in amanner and at a rate such that the reaction or precipitation is carriedout at the preset, constant pH. After the pigment has been completelyprecipitated and depending upon the ultimate or intended use of thepigment the pH can be reduced or adjusted by the addition of acid, etc.While the product of the aforesaid application Ser. No. 285,966 has highabrasiveness and can be advantageously used in dentifrice compositions,it has unexpectedly been discovered that products produced in accordancewith the method of the instant invention have significantly improvedcleaning ability or action, with the latter being the same as or equalto that of high grade phosphate toothpaste polishing agents. In additionand as previously noted the products, when used in toothpasteformulations, have a fluoride compatibility (i.e., the fluoride isavailable in its soluble form and not converted for example, to aninsoluble salt, etc.) that is superior to, and not subject to thedisadvantages of, known phosphates, such as calcium pyrophosphate.

As will be seen from the above and turning now to further specificdetails in the practice of the invention, the starting materials includean electrolyte; an alkali metal silicate and an acid. As used herein theterm alkali metal silicate includes all the common forms of alkalisilicates, as for example, metasilicates, disilicates and the like.Water soluble potassium silicates and sodium silicates are particularlyadvantageous. Because of their relatively low cost, sodium silicates arepreferred. Sodium silicates are effective in any composition in whichthe mole ratio of the SiO₂ to Na₂ O is from about 1.0 to 4. In thisregard commercially available sodium silicate solutions are more or lesspolymerized depending on their silica to sodium oxide (SiO₂ /Na₂ O)ratios. For example, sodium metasilicate solution (mole ratio unity) isknown to be predominantly monomeric in character while water glass (moleratio 3.3) is both monomeric and polymeric in character. As the silicato sodium oxide mole ratio of sodium silicate increases, so does thepolymer to monomer ratio of its silicate anions. While sodium silicateshaving an SiO₂ /Na₂ O mole ratio of from 1 to 4 may be employed,particularly advantageous results are obtained if the SiO₂ /Na₂ O ratiois in the range of from about 2.0 to 3.3, or more preferably from about2.0 to 2.8.

Although the commercially available silicate solutions may be more orless polymerized depending on their silica to sodium oxide (SiO₂ /Na₂ O)ratios, it has also been found that particularly advantageous andpreferred results are obtained if a sulfate salt or electrolyte is addedto the silicate solution prior to its simultaneous introduction with theacid into the water-electrolyte reaction or receiving medium. In otherwords the silicate solution itself can be pre-polymerized as per theteachings of U.S. Ser. No. 285,966. In addition, and in lieu of simplymixing the salt with the silicate solution, the silicate itself may be aproduct such as disclosed in U.S. application Ser. No. 193,485 whereinsilica is reacted with a hydroxide in the presence of a sulfate salt toproduce a polysilicate.

While the acidulating agent or acid is preferably a strong mineral acid,such as sulfuric acid, nitric acid and hydrochloric acid, it should beunderstood that other acids, including organic acids, as for example,acetic acid, formic, or carbonic acid, and salts of carbonic acid suchas ammonium carbonate, can be employed. As noted above, the acidulatingagent and silicate should preferably be added as dilute solutionsthereof. Preferred results are obtained if the acidic solution is fromabout 10 to 25% by weight acid based on the total weight of thesolution. Particularly advantageous and thus preferred results areobtained if the concentration of the silicate solution is on the orderof from about 1.0 to about 2.5 pounds per gallon.

As known in the art, the term electrolyte refers to ionic or molecularsubstances which, when in solution, break down or disassociate to formions or charged particles. As used herein (and referring to theelectrolyte added to the water prior to the introduction of the acid andsilicate solutions) the term electrolyte is intended to have its commonmeaning. However in the practice of the invention, and again asdiscussed above, the electrolyte should be compatible with the acid andsilicate. Thus if sodium silicate and sulfuric acid are employed thepreferred electrolyte would be sodium sulfate. If hydrochloric acid isemployed as the acid (again using sodium silicate) then the electrolytewould be sodium chloride. If the silicate-acid combination is potassiumsilicate and sulfuric acid, then the electrolyte would be potassiumsulfate, etc. Further examples of electrolytes include sodium nitrate,sodium acetate and the like. It should be noted however that theelectrolyte must not be a salt that would produce a water insolubleby-product with the precipitating pigment. An example of this is calciumsulfate. Thus a calcium salt could not be used with sulfuric acid. Theamount of the electrolyte used should be in the range of from 3 to 15%by weight, based on the weight of the water (receiving medium) initiallyadded to the reaction. In general the temperature employed inprecipitating the pigment of the invention is not critical and is thesame as in the above discussed known processes. In a preferredembodiment the receiving medium containing the electrolyte is heated toa temperature in the range of from 100° to 200°F prior to theintroduction of the silicate and acid.

As should be readily appreciated by those skilled in the art, no specialequipment is required in the method herein described. In this regard,however, the reactor should be equipped with heating means, e.g., asteam jacket, in order to maintain the desired reaction temperature andshould have adequate agitating means to produce a strong backflow on thebody of the liquid and thus avoid zones of high concentration of theincoming reactants. It is desirable to bring the reactants together soas to produce an instantaneous reaction of all material being fed to thefullest extent reasonably possible, as such promotes uniformity of theresulting products. Storage vessels (for the reactants) connected to thereaction vessel through lines fitted with flow control means may also beprovided. The reaction vessel may be equipped with an outlet lineleading to a filter which may be of conventional design. Afterprecipitation, the filtered mass is washed and dried. Such steps mayalso be conducted in conventional equipment, it being understood, ofcourse, that same do not form a part of the present invention.

If the pigments of the invention are used in toothpaste compositions,the dentifrice (if in the form of a paste) may contain humectantmaterials and binders to give the dentifrice a smooth texture and goodflowability. Glycerine, sorbitol, corn syrup, glucose and the like maybe used as carriers. Examples of binders include gum tragacanth, sodiumcarboxymethylcellulose and the like. The above materials as well asspecific formulations and ingredients of toothpaste compositions arewell known in the art and are disclosed in numerous publications andpatents e.g., in U.S. Pat. Nos. 2,994,642 and 3,538,230.

Before turning to specific examples, it may be noted that as used hereinthe term "structure" is intended to include, and is defined as, theability of a silica material to hold water in its wet cake. Whensilicas, such as the aforementioned conventional precipitated silicas,hold a high percentage of water, i.e., in the neighborhood of 75 to 85%,they are known and referred to as high structure silicas. Materialsholding less than 75% and preferably in the neighborhood of from about50 to 70% water in their wet cake are referred to as low structuresilicas.

The invention will be further illustrated by the following exampleswhich set forth particularly advantageous method and compositionembodiments. While the examples serve to illustrate the presentinvention, they are not intended to limit it thereto.

EXAMPLE 1

In this experiment dry sodium sulfate was added to 10.0 gallons of waterin a 200 gallon reactor such that the sodium sulfate concentration inthe reaction medium was 10%. The pH of the reaction medium was adjustedto 9.0 by the addition of sodium silicate. The reaction medium was thenheated to 150°F. Sodium silicate having an SiO₂ to Na₂ O mol ratio of2.5 and a concentration of 2.0 pounds/gallon and sulfuric acid of 11.4%concentration were then added to the reaction medium at the rate of 756ml/min and 453 ml/minute respectively so that a constant precipitationpH of 9.0 was maintained. The sodium silicate solution employed in thisExample also contained 7% sodium sulfate which was added to the solutionprior to its introduction into the reactor. After 30 minutes theprecipitation was complete. Excess acid was added until a slurry pH of5.4 was reached. The reaction slurry was digested at 170°F for 20minutes and then filtered, washed, dried and milled in the conventionalmanner. The product in this Example had a wet cake moisture content of51%; a surface area of 173 m² /g; a pack density of 35.6 lbs/cu.ft. anda valley abrasion (mg wire loss) of 70.2. It was further found that theproduct had a relative cleaning scale (RCS) of 100, which was the sameas the phosphate polishing agent disclosed in U.S. Pat. No. 3,359,170,and had superior fluoride stability.

EXAMPLE 2

Example 1 was repeated except that the reaction temperature wasmaintained at 175°F and the precipitation pH was 10.0. Digestiontemperature was 199°F. The batch was otherwise processed in theconventional manner. The properties of the product were substantiallythe same as in Example 1.

EXAMPLE 3

Example 1 was repeated except that the reaction temperature wasmaintained at 185°F and the batch was digested at 199°F. The batch wasotherwise processed in the conventional manner. The properties of theproduct were substantially the same as in Example 1.

EXAMPLE 4

In a series of tests the general procedure of Examples 1-3 were repeatedexcept that the precipitating pH, although held constant in each test,was varied from 8.0 to 10.4. The results were substantially the sameexcept that it was found that the specific properties could becontrolled within predetermined limits by changing the pH. In any eventthe products had a low wet cake moisture content, were of low structure,had relatively low oil absorptions and higher valley abrasion ascompared to a control wherein conventional precipitated silica wasprepared by neutralizing 1.24 lbs/gal sodium silicate (10 gal.) with11.4% sulfuric acid. The valley abrasion of the latter was found to be2.5 as compared to up to 167.8 (Example 3) of the products of theinvention.

EXAMPLE 5

The general procedures of Example 1-4 were repeated except that nitricacid, hydrochloric acid, acetic acid and formic acid were substitutedfor the sulfuric acid. The results were substantially the same as inExamples 1-4. The corresponding salts (e.g., sodium nitrate, sodiumchloride, etc.) were also used as the electrolyte initially charged tothe reactor (in lieu of the sodium sulfate).

EXAMPLE 6

In a series of tests the general procedures of Examples 1-5 wererepeated except that aqueous sodium silicates having mol ratios (SiO₂/Na₂ O) in the range of from 1 to 3 were substituted for the 2.5silicate of Examples 1-5. The results were generally the same as inExamples 1-5 except that it was found that the use of alkali metalsilicates having an SiO₂ /Na₂ O mol ratio within the range of from about2.0 to 2.8 resulted in superior properties (as defined hereinabove).

EXAMPLE 7

The procedures of Examples 1-6 were repeated except that the saltemployed as the electrolyte in the aqueous receiving medium was variedfrom 3 to 15% by weight (by increments of about 5%). The results weresubstantially the same as in the above Examples.

EXAMPLE 8

The procedure of Example 1 was repeated except that the sodium silicatesolution did not contain any sulfate prior to its introduction into thereactor. It was found that while the valley abrasion was relatively highas compared to the control (see Example 4) the cleaning action was about85 (RCS) as compared to 100 for the product of Example 1.

From the above Examples it is clear that the process of the inventionresults in silicas of lower wet cake moisture, lower structure, loweroil absorption, higher pack density and higher valley abrasion than theconventional products. In addition (and this was truly unexpected) thecleaning action of the new products when used in toothpaste were foundto be very high and equal to known phosphates. The cleaning action wasalso approximately 50% higher than the products produced in accordancewith the teachings of U.S. application Ser. No. 285,966. The valleyabrasion of the latter and the products of the invention are similar.

The new process also leads to silicas of lower processing costs thanregular precipitated silicas. For example, the average wet cake moisture(Examples 1 through 8) of silicas produced via the new process isapproximately 53% as opposed to 82% for regular silica. This means 47parts of dry silica from 100 parts of wet cake can be recovered. Thus,via the new process 29 parts of more dry silica or an increase of(29/18) × 100 or about 160% is obtained. The new process results insilicas of better drying and filtration rates and hence significantlylower processing costs than the precipitated silicas produced by theconventional process.

From the above it will be seen that the present invention provides atruly remarkable and simplified process for producing silicic acidpigments having new and unique properties. While preferred method andcomposition embodiments have been disclosed for illustrative purposes itshould be understood that the invention is not limited thereto. Forexample, as taught in U.S. application Ser. No. 285,966, the refractiveindex of the precipitated pigment can be controlled by the addition ofan adduct element (such as aluminum, magnesium and the like) to providean abrasive or polishing agent for a clear translucent or transparenttoothpaste composition. In this embodiment, the acid is premixed with asolution of the adduct material (e.g., aluminum sulfate) and theacid-metal salt mixture is then used for acidulating the alkali metalsilicate.

As used herein, the term "pigments" is not intended to be limited tocolor-bearing materials which impart color to other substances ormixtures, but is intended to refer to the finely divided, powdery natureof the materials, such as silica, silicon dioxide, precipitated silica,silica abrasives, sodium alumino silicates, and the like, which are alsoin other contexts referred to as fillers, extenders, and reinforcingpigments.

What is claimed is:
 1. A method for producing amorphous, precipitated silicic acid pigments having improved chemical and physical properties when employed in dentifrice compositions, said method comprising the steps of: providing an aqueous reaction medium containing an electrolyte, adjusting the pH of the aqueous medium to a value in the range of from about 8.0 to 10.4; simultaneously introducing an acid and a prepolymerized alkali metal silicate into said aqueous reaction medium; said acid being selected from the group consisting of sulfuric acid, nitric acid and hydrochloric acid; said prepolymerized alkali metal silicate being selected from the group consisting of sodium silicate, potassium silicate and lithium silicate, said alkali metal silicate being at least partially polymerized and having an SiO₂ /X₂ O ratio of about 2.0 to 3.3, wherein X is selected from the group consisting of sodium, potassium and lithium, and said electrolyte comprises the sodium, potassium or lithium salt of said acid; continuing the addition of the said acid and said silicate to said aqueous reaction medium in a manner such that a substantially constant pH in the range of from about 8.0 to 10.4 is maintained and until the precipitation of said pigment is substantially complete and recovering the precipitated pigment from the aqueous reaction medium.
 2. The method in accordance with claim 1 wherein said alkali metal silicate has an SiO₂ /X₂ O mol ratio in the range of from about 2.0 to 2.8, wherein X is selected from the group consisting of sodium, potassium, and lithium.
 3. The method in accordance with claim 1 wherein said electrolyte is added to the aqueous reaction medium in an amount in the range of from 3 to 15% by weight based on the weight of the reaction medium.
 4. The method in accordance with claim 1 wherein the concentration of the solution of said silicate is in the range of from about 1.0 to 2.5 pounds per gallon and the acidic solution is from about 10 to 25% by weight acid based on the total weight of acidic solution.
 5. The method in accordance with claim 1 wherein an adduct material selected from the group consisting of the water soluble salts of aluminum and alkaline earth metals is combined with said acid to thereby control the refractive index of the precipitated pigment and to form a metal silicate pigment having particular utility for use as an abrasion and gelling agent in clear tooth compositions. 